Re: [PATCH] xfs: handle array index overrun in xfs_dir2_leaf_readbuf()

Darrick J. Wong
2017-04-19T00:29:32Z

On Tue, Apr 18, 2017 at 07:14:03PM -0500, Eric Sandeen wrote:
> On 4/18/17 7:09 PM, Darrick J. Wong wrote:
> > On Tue, Apr 18, 2017 at 12:55:44PM -0400, Brian Foster wrote:
> >> On Mon, Apr 17, 2017 at 05:12:36PM -0500, Eric Sandeen wrote:
> >>> On 4/17/17 3:57 PM, Brian Foster wrote:
> >>>> On Thu, Apr 13, 2017 at 01:45:43PM -0500, Eric Sandeen wrote:
> >> ...
> >>>>
> >>>> This fix seems fine to me, but I'm wondering if this code may have
> >>>> issues with other kinds of misalignment between the directory blocks and
> >>>> underlying bmap extents as well. For example, what happens if we end up
> >>>> with something like the following on an 8k dir fsb fs?
> >>>>
> >>>> 0:[0,xxx,3,0]
> >>>> 1:[3,xxx,1,0]
> >>>>
> >>>> ... or ...
> >>>>
> >>>> 0:[0,xxx,3,0]
> >>>> 1:[3,xxx,3,0]
> >>>
> >>> Well, as far as that goes it won't be an issue; for 8k dir block sizes
> >>> we will allocate an extent map with room for 10 extents, and we'll go
> >>> well beyond the above extents which cross directory block boundaries.
> >>>
> >>>> ...
> >>>> N:[...]
> >>>>
> >>>> Am I following correctly that we may end up assuming the wrong mapping
> >>>> for the second dir fsb and/or possibly skipping blocks?
> >>>
> >>> As far as I can tell, this code is only managing the read-ahead state
> >>> by looking at these cached extents. We keep track of our position within
> >>> that allocated array of mappings - this bug just stepped off the end
> >>> while doing so.
> >>>
> >>> Stopping at the correct point should keep all of the state consistent
> >>> and correct.
> >>>
> >>> But yeah, it's kind of hairy & hard to read, IMHO.
> >>>
> >>> Also as far as I can tell, we handle such discontiguities correctly,
> >>> other than the bug I found. If you see something that looks suspicious,
> >>> I'm sure I could tweak my test case to craft a specific situation if
> >>> there's something you'd like to see tested...
> >>>
> >>
> >> Background: Eric and I chatted a bit on irc to rectify that what I'm
> >> calling out above is a different issue from what is fixed by this patch.
> >>
> >> Eric,
> >>
> >> I managed to construct a directory that looks like this:
> >>
> >> EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL
> >> 0: [0..7]: 88..95 0 (88..95) 8
> >> 1: [8..15]: 80..87 0 (80..87) 8
> >> 2: [16..39]: 168..191 0 (168..191) 24
> >> 3: [40..63]: 5242952..5242975 1 (72..95) 24
> >>
> >> The fs has 8k directory fsbs. Dir fsb offset 0 spans extents 0 and 1,
> >> offset 1 (which corresponds to the 512b range 16-31 above) is covered
> >> completely by extent 2 and dir offset 2 (range 32-47) spans extents 2
> >> and 3. An ls of this directory produces this:
> >>
> >> XFS (dm-3): Metadata corruption detected at xfs_dir3_data_reada_verify+0x42/0x80 [xfs], xfs_dir3_data_reada block 0x500058
> >> XFS (dm-3): Unmount and run xfs_repair
> >> XFS (dm-3): First 64 bytes of corrupted metadata buffer:
> >> ffffbcb901c44000: 00 00 00 00 00 00 00 64 0f 78 78 78 78 78 78 78 .......d.xxxxxxx
> >> ffffbcb901c44010: 78 78 78 78 2e 38 38 36 01 00 00 00 00 00 10 00 xxxx.886........
> >> ffffbcb901c44020: 00 00 00 00 00 00 00 64 0f 78 78 78 78 78 78 78 .......d.xxxxxxx
> >> ffffbcb901c44030: 78 78 78 78 2e 38 38 37 01 00 00 00 00 00 10 20 xxxx.887.......
> >>
> >> ... which is yelling about block 184 (dir fsb 2). The fs is otherwise
> >> clean according to xfs_repair.
> >>
> >> I _think_ something like the appended diff deals with it, but this is
> >> lightly tested only and could definitely use more eyes.
> >>
> >> Brian
> >>
> >> --- 8< ---
> >>
> >> diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
> >> index ad9396e..9fa379d 100644
> >> --- a/fs/xfs/xfs_dir2_readdir.c
> >> +++ b/fs/xfs/xfs_dir2_readdir.c
> >> @@ -404,7 +404,8 @@ xfs_dir2_leaf_readbuf(
> >> * Read-ahead a contiguous directory block.
> >> */
> >> if (i > mip->ra_current &&
> >> - map[mip->ra_index].br_blockcount >= geo->fsbcount) {
> >> + (map[mip->ra_index].br_blockcount - mip->ra_offset) >=
> >> + geo->fsbcount) {
> >> xfs_dir3_data_readahead(dp,
> >> map[mip->ra_index].br_startoff + mip->ra_offset,
> >> XFS_FSB_TO_DADDR(dp->i_mount,
> >> @@ -432,7 +433,7 @@ xfs_dir2_leaf_readbuf(
> >> * The rest of this extent but not more than a dir
> >> * block.
> >> */
> >> - length = min_t(int, geo->fsbcount,
> >> + length = min_t(int, geo->fsbcount - j,
> >
> > Looks ok to me to make Eric's bugfix complete.
>
> Brian, thanks for crafting the image to expose this. :)
>
> I've been otherwise occupied today, sorry - technically this fixes a separate
> issue, yes? So 2 patches, 2 bugfixes AFAICT?

Ok. I don't mind taking two patches since you're right, the first patch
saves us from running off the end of the shadow bmap array, and the
second one fixes bookkeepping problems when a dir extent starts midway
through a large dirblock.

--D

>
> Thanks,
> -Eric
>
> > I will, however, post a cleanup patch to remove the persistent shadow
> > bmap and have readahead issued directly off the inode fork contents.
> >
> > --D
> >
> >> map[mip->ra_index].br_blockcount -
> >> mip->ra_offset);
> >> mip->ra_offset += length;
> >> --
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